1. Field of the Invention
The invention relates to transparent substrates, in particular those made of glass, coated with a stack of thin layers including at least one metal layer having reflective properties on long-wavelength infrared radiation and/or on solar radiation.
The invention also relates to the use of such substrates in the manufacture of thermal-insulation and/or solar-protection glazing, denoted hereinbelow by the term "functional" glazing. Such glazing may be used on both buildings and vehicles, especially for the purpose of decreasing the air-conditioning load and/or for reducing excessive overheating resulting from the presence of expansive glazed surfaces in rooms and passenger compartments.
2. Description of the Related Art
In the past, two approaches have been proposed for improving the thermal, antisolar or low emissivity properties of a stack of layers with reflective properties:
The first solution, which is the simplest, is to increase the thickness of the functional layer in order to lower the emissivity or increase the anti-solar capacity of the functional layer. However, this solution, has until the present invention had disadvantageous repercussions with respect to the optical appearance of the substrate carrying the stack, since it results in a decrease in its light transmission (T.sub.L) and, above all, a more marked coloration in reflection, and usually, in tints which are unattractive. A limit in terms of thickness of the functional layer is therefore rapidly reached, beyond which the substrate no longer satisfies the desired optical criteria. PA1 The second solution is to improve the quality of the "functional" layer, for a given thickness, through better crystallization which makes the layer less absorbent. This may be achieved by arranging below the functional layer, a particular type of dielectric material which promotes its growth or its "wetting". Thus, European Patent Application EP-A-0,611,213 recommends a wetting coating based on niobium or tantalum oxide below the low-emissivity silver layer, while French Patent Application 94/04810 filed on Apr. 21, 1994, published under the number FR-A-2,719,036 and corresponding to European Patent Application EP-A-0,678,484, describes a "double" wetting coating below the low-emissivity layer and consists of a first layer of niobium or tantalum oxide surmounted by a second layer of zinc oxide. This approach is limited because the quality of the functional layer improved by these subjacent layers reaches a maximum level which is difficult to exceed, for a given layer thickness.